Neutron spectral fluence measurements using a Bonner sphere spectrometer in the development of the iBNCT accelerator-based neutron source

The neutron spectral fluence of an accelerator-based neutron source facility for boron neutron capture therapy (BNCT) based on a proton linac and a beryllium target was evaluated by the unfolding method using a Bonner sphere spectrometer (BSS). A ³He-proportional-counter-based BSS was used with weak beam during the development of the facility. The measured epithermal neutron spectra were consistent with calculations. The epithermal neutron intensity at the beam port was estimated and the results gave a numerical target for the enhancement of the proton beam intensity and will be used as reference data for measurements performed after the completion of the facility. - Highlights: • The neutron spectral fluence of an accelerator-based BNCT facility was evaluated. • A ³He-based BSS was used with weak beams, only available in the developing facility. • Results were consistent with the Monte Carlo calculated neutron energy distributions. • Results are useful to determine the target proton beam intensity for clinical use.